The Hoover Dam may be the Eighth Wonder of the World, but to me the more impressive achievement has always been Lake Mead, the man-made reservoir—which can contain nearly 10 trillion gallons of water—that the dam holds back. Lake Mead is a vast, living tank of water in the middle of the Nevada desert, as unexpectedly remarkable as Las Vegas itself. But the lake is also a keystone in the complex irrigation system that keeps the parched states of the American Southwest wet with the waters of the Colorado River. Las Vegas gets 90% of its water from Lake Mead, and the sight of the rocky reservoir filled to the brim has always been a reassuring sign for a town built on luck.

But those days are long gone. The past decade has seen a precipitous decline in Lake Mead’s water levels, as a stubborn, multi-year drought and continued growth in the Southwest have combined to drain the reservoir. The jagged rocks that form the boundaries of the reservoir are marked with bathtub rings, showing how far the water has fallen from the high point. And it’s getting worse—as Felicity Barringer of the New York Times reports, some time this Sunday morning the water level at Lake Mead fell lower than it has ever has since it was first filled during the construction of the Hoover Dam 75 years ago. On Sunday the water level had dropped to 1,083.18 ft, and it had fallen further to 1,083.09 ft this morning. As this historical graph (PDF) from the Bureau of Reclamation shows, Lake Mead is entering uncharted territory—a thought that can’t be comforting for the millions of people who depend on the lake to fill their faucets. As Barry Nelson, a senior policy analyst for the Natural Resources Defense Council, told Barringer:

This strikes me as such an amazing moment. It’s three-quarters of a century since they filled it. And at the three-quarter-century mark, the world has changed…This is the place where the mega-dam began, and it may be the place where it ends [because of] climate change and new constraints on water supplies.

What’s causing Lake Mead to dry up—and what does it mean for the Southwest? The one undeniable cause is simple growth—Las Vegas has grown from 25,000 people in 1950 to some 2 million today. That means more lawns, more laundry, more swimming pools, more car washes—in general, more straws sucking the water out of Lake Mead. And of course Las Vegas isn’t the only area in the Southwest to experience booming growth over the past few decades. From Denver to Phoenix to Los Angeles, the once lightly populated West has exploded, even as farmers in the region draw more water from the system to irrigate the desert.

But the Southwest has also been caught in a devastating drought that has now gone on for more than 10 years, one that has reduced the region’s water supplies even as growth has further stressed them. Drought is a natural phenomenon—especially in the desert, go figure—and there have been varying levels of rainfall in the region just in the 75 years since Lake Mead was first filled. But the scary thing is that the territory might be more vulnerable to drought than it seemed during the 20th century—a time period that may have been unusually wet on a historical scale. A 2007 panel organized by the National Research Council found evidence that mega-droughts had occurred in the Southwest more frequently than had been thought, and that “drought episodes are a recurrent and integral feature of the region’s climate.” The Colorado River Compact—which divides up water supplies for seven U.S. states and parts of Mexico—was drawn up in 1922, based on river flow data going back to the 1890s, a time of unusual wetness. We may have built the Southwest with a false sense of water security.

Then there’s climate change, an X factor for future water supplies. It’s difficult to gauge what impact, if any, global warming may have had on the current drought and on dropping water levels. As always, it’s virtually impossible to filter out climate change as a cause for a natural disaster amid all the noise and static of other factors. But as the recent report from the government U.S. Global Change Research Program shows, the Southwest is already rapidly warming, reducing the spring mountain snowpack that helps feed the rivers of the region. We’re likely to see increasing temperatures in the future, with more frequent drought and increasingly scarce water supplies. (See the rather alarming graph above.) Climate change won’t be the only cause behind the drying of the West, but could make a bad situation much, much worse.

As it stands now, Lake Mead only has to drop a few more feet—to 1,075 ft—before a new water distribution system would kick in, reducing deliveries to Arizona and Nevada. It’s possible that water managers may be able to avoid that catastrophe by diverting additional supplies from Utah’s Lake Powell upriver, which contains about 50% more water than Lake Mead does. (Powell has risen more than 60 ft. since 2004, in part because the upper-basin states on the Colorado River don’t use their full allocation, as opposed to thirsty states like Arizona and Nevada.) You can count on that plan going forward—if Lake Mead’s water level were to drop beneath 1,050 ft., it might become impossible for Hoover Dam’s hydroelectric turbines to work, turning a water shortage into a power panic as well. But what’s clear is that in a hotter, drier future, the Southwest will have decisions to make about how it can best husband its limited water resources. A West of burgeoning cities and suburbs and a West of irrigated farms may no longer be able to coexist.